FIG. 5 is a cross-sectional view of a conventional semiconductor laser including an active layer with a multiple quantum well structure. That laser includes an n-type GaAs substrate 1 and a double heterojunction structure disposed on the substrate. The double heterojunction structure includes an n-type AlGaAs first cladding layer 2, an AlGaAs active layer 3 including a multiple quantum well structure, and a p-type AlGaAs second cladding layer 4. The semiconductor laser also includes a p-type GaAs contact layer 5 disposed on the second cladding layer 4 and electrodes 6 and 7 disposed on the substrate 1 and the contact layer 5, respectively. Opposed facets 8 and 9 of the semiconductor laser that form a resonator with the active layer and the first and second cladding layers are transverse to those layers. The active layer 3 includes alternatingly arranged well and barrier layers with relatively low and relatively high energy band gaps, respectively.
When the semiconductor laser is forward biased, electrons and holes are injected into the active layer and recombine to produce light. When the current flowing between the electrodes exceeds a threshold current level, laser oscillation occurs and coherent light is produced by the laser. The light is emitted through one of the facets 8 and 9 that is coated with an imperfect reflector. Because of the reflection of the laser light within the active layer at the facets and the presence of surfaces states at the facets, during operation of the laser some of the light is absorbed at and adjacent the facets, locally increasing the temperature of the laser. If the temperature rises sufficiently, COD can occur in which semiconductor materials may begin to melt, destroying the semiconductor laser.
In order to increase the power output that a semiconductor laser can safely produce without COD, so-called window structures have been included in semiconductor lasers at the facets. Those window structures generally have an effective energy band gap larger than other parts of the active layer so that light is less easily absorbed at the facets than internally in the laser. In semiconductor lasers including a multiple quantum well structure in the active layer, a known technique for producing window structures includes disordering the multiple quantum well structure in the vicinity of the facets. When the multiple quantum well structure is disordered, the distinct boundaries between the well and barrier layers are blurred, resulting in a more homogeneous active layer at the facets. The more homogeneous material has a larger energy band gap than the effective energy band gap of the quantum well structure so that light absorption is reduced. Disordering may be produced thermally or by the introduction of impurities in controlled regions of the active layer adjacent the facets. However, in disordering the multiple quantum well structure, it is sometimes difficult to control the degree and location of the disordering, resulting in the production of defective semiconductor lasers.